Quinone-sensitized Steady-state Photolysis of Acetophenone Oximes Under Aerobic Conditions: Kinetics and Product Studies[dagger]

Photochemistry and Photobiology, Jan/Feb 2006 by Park, Adriana, Kosareff, Nicole M, Kim, Jason S, de Lijser, H J Peter

In our earlier studies (27) we noted that analysis of the data yielded better results when the meta- and the para-substituted compounds were separated. Similar results were obtained this time (Table 2). Significant improvements are noticeable when dual correlations are used (Fig. 4). For example, using the polar and radical substituent constants gives correlations of 0.93 for the meta data and 0.98 for the para data. The contribution of the radical constants is remarkable, especially for the meta-substituted acetophenone oximes. In this case, ρ^sub rad^/ρ^sub pol^ = - 5.4, which is similar to the previously (LFP) observed ratio of -4.2. For the para-substituted oximes the polar and radical contributions are approximately equal (ρ^sub rad^/ρ^sub pol^ = -1.05). Analysis of the data with the use of the Jiang and Ji polar (σ^sub mb^) and radical (σ^sup *^^sub JJ^) substituent constants (41) gives similar results for the para data set (ρ^sub JJ^/ρ^sub mb^ = -1.9; r^sup 2^ = 0.95), but the correlation for the meta data set is significantly worse (r^sup 2^ = 0.39), although the importance of radical effects is seen again (ρ^sub JJ^/ρ^sub mb^ = -6.3).

It is interesting to note that for the para data only two substituents are significantly influenced by the radical substituent coefficients. Comparing Figs. 4B,C clearly shows that both p-NO^sub 2^ and p-CN acetophenone oximes deviate from the line when a polar substituent coefficient is used. Without these data points, the correlations would be 0.91 (ρ^sub mb^ = -0.91) and 0.86 (ρ^sub pol^ = -1.30). These results are consistent with those from the dual-parameter correlations; for the para-substituted acetophenone oximes, polar effects are more important than radical effects.

Overall, these observations are in reasonable agreement with those of the earlier LFP (quenching) studies (Table 2). The most obvious difference is the observation that in the LFP studies good correlations are often obtained with single parameters, whereas in the SS experiments dual-parameter correlations are necessary in order to observe clear trends. This is in agreement with the fact that in the LFP experiments only the first step is important and a single species (with both radical and polar character) is generated. In the SS experiments, there are a number of follow-up steps involving several types of intermediates, each of which are influenced by polar and radical effects in different ways.

Product ketone formation in the photosensitized reactions of meta- and para-substituted acetophenone oximes

The CA-sensitized reactions resulted in the formation of the corresponding ketone as the major product and a minor side product tentatively identified as the (l-chloroethyl)aryl derivative. In the case of p-methoxyacetophenone oxime, the major product was found to be the corresponding amide; this unusual observation is further discussed below. The use of CA as a sensitizer in PET reactions is well established (44). Triplet chloranil (^sub 3^CA) is a powerful oxidant (E*^sub red^ = 2.15 V). capable of oxidizing a wide variety of substrates, including arenes, alkenes, alcohols, and amines (45-48). However, one early report has suggested the formation of HCI in the photosensitized reactions of CA with aldehydes (49). Clearly, formation of HCI under our conditions could be responsible for the observed products as well. To rule out the involvement of HCl in these reactions, the irradiations were also carried out with other sensitizers such as benzophenone and 9,10-dicyanoanthracene (DCA). In both cases, the oxime was converted into the corresponding carbonyl compounds. These results, together with those discussed above, strongly support an initial electron transfer step.